The present invention relates to apparatus for bending tubes and more particularly concerns an apparatus having an improved clamp die arrangement.
A common form of bending tubes, employing either compression or draw bending, involves clamping of a forward section of a tube to a rotary bend die by a clamp die and clamping a rearward section of the tube against the bend die by a pressure die. The bend and clamp dies are rotated together, with the tube clamped therebetween, to bend the tube around the bend die. In compression bending, little or no axial restraint is applied to the tube, which thus moves axially as the bend die rotates. In draw bending, sufficient axial restraint is applied to the tube to stretch the tube beyond its elastic limit as it bends around the bend die, thus minimizing wrinkling or buckling of tube material on the inside of the bend. A combination of compression and draw bending is described in my co-pending applications identified above.
As described in such co-pending applications, draw bending requires a sufficient axial restraint to be exerted upon the tube by a pressure die so that the material of the tube can be stressed past its yield point. This is achieved by exerting sufficient pressure on the tube by the pressure die and, concomittantly, by the clamp die, which presses a forward portion of the tube against the bend die. In order to achieve adequate pressure on the tube (between the clamp and bend die) for draw bending, the clamp die must press against the tube over a significant length of the tube. Commonly the clamp die has a length, parallel to the extent of the tube, in the order of three times the tube diameter. If the clamp die is much shorter than this, the tube is likely to slip relative to the clamp die or, in the alternative, such great force must be exerted by the clamp die upon the tube that the tube is often unacceptably deformed. Because large clamp dies have been required for draw bending, it has not been possible in this type of bending to form tube bends close together. Two successive bends cannot be any closer to each other than the length of the clamp die since the latter, in prior machines, must operate upon a straight and as yet unbent portion of the tube.
In my prior applications above identified, a method of bending is described in which the required length of the clamp die is greatly minimized by starting a bend as a compression bend, without axial restraint, and then, after the tube has been partly bent around the bend die, employing the frictional forces between the bent tube and the bend die to augment the clamping force of a short clamp die and to enable it to exert the axial restraint required for the subsequent draw bending which completes the bend.
However, even with the greatly shortened clamp die of my prior invention, the die is capable of operation only upon a section of the tube that is straight and as yet unbent. Thus, a portion of the tube to the rear of a prior bend must be provided for cooperation with the clamp die and such tube portion must be straight and unbent for at least the length of the clamp die. In my prior invention, even though the required length of the clamp die is considerably decreased, one bend still cannot be made any closer to a succeeding bend than the length of this shortened clamp die. Stated otherwise, there must be provided (in prior arrangements) a straight tube portion between two successive bends, which straight portion has a length equal to that of the clamp die.
Accordingly, it is an object of the present invention to provide a bending machine in which the length of a required straight portion of tube between successive bends is minimized.